Function build_expression_context 

pub fn build_expression_context(
    state: &Value,
    item: Option<&Value>,
) -> HashMap<String, Value>

Build a context for expression evaluation from state and optional item

Flattens state and item objects so that expressions can access:

• state.user.name via path traversal
• item.selected via path traversal
• Direct access like selected if we inject item properties directly